The Mossback’s Northwest production team met in Thurston County last week to research some geological history. We learned that the Missoula Floods that scoured Eastern Washington weren’t the only massive, earth-shaping floods at the end of the last Ice Age some 15,000 years ago. The retreating Puget Lobe — the last ice sheet’s southern extension, which stopped just south of Olympia — sent massive flows south and west as it melted.
But while retreating, the ice also blocked rivers flowing from Mount Rainier, like the Carbon and Puyallup. This created a series of glacial lakes which, when unblocked, periodically let loose with floods and debris flows, shaping the valleys and prairies of the region. We talked at length with geologist Patrick Pringle who has, with his research partner Barry Goldstein, studied this phenomenon. As a result of the floods, Pringle says, there is a lot of volcanic rock — from gravel to massive boulders — in the valleys of South Puget Sound that washed down from Tahoma. More on this is coming in Season 11 this fall.
Reprinted from the Cascade PBS ‘The Mossback Den’ newsletter